1. Field
Embodiments of the present invention may relate to a mobile communication apparatus and method. More particularly, embodiments of the present invention may relate to an apparatus and method for transmitting and receiving a random access channel (RACH) signal in a single carrier-frequency division multiple access (SC-FDMA) system.
2. Background
A common channel may be used to request for uplink synchronization and data traffic channels. Such a common channel may be defined as a RACH. A RACH signal may include a preamble portion and a message portion. The base station may use 1 bit preamble information received from the mobile station to determine the presence of a RACH message and to calculate timing offsets (receive delay). In response to receiving the preamble from the mobile station, the base station transmits an ACK (ACKnolwedge) to the mobile station. In response to receiving an ACK from the base station, the mobile station transmits the RACH message to the base station. The RACH message may include a message in an upper layer for the mobile station to connect to the network. The message may include, for example, the mobile station's identification, the purpose of connection to the network, priorities, etc. If the mobile station transmits the RACH message to the base station after a predetermined time or after the mobile station receives an ACK from the base station, then the base station may not obtain information related to the mobile station until it receives the RACH message. Therefore, the base station has to predict performance of the mobile station if it has to transmit any messages to the mobile station before receiving the RACH message.
The RACH signal may be transmitted and received in a code division multiple access (CDMA) method for a wideband CDMA (WCDMA) system and in an orthogonal frequency division multiple access (OFDMA) method for worldwide interoperability for microwave access (WiMax) system. These two methods are briefly discussed below.
FIG. 1 illustrates a RACH signal transmission for a CDMA system according to one arrangement. A code is allocated to a RACH since channels are divided by their unique codes in the CDMA system. A preamble of the RACH signal may include the allocated code. As the number of codes for the preamble increases, the likelihood for mobile stations to conflict decreases. As shown in FIG. 1, a mobile station transmits a first preamble 120 to a base station. If the mobile station does not receive an ACK for the first preamble 120 from the base station, then the mobile station increases the transmitting power by Δ in order to transmit a second preamble 140. If the mobile station does not receive an ACK for the second preamble 140 from the base station, then the mobile station increases the transmitting power by Δ once again in order to transmit a third preamble 160. Such a power control is referred to as “preamble power ramping.” This is because each user's signal may create interference noise to other users in the CDMA system, thereby requiring communication in minimum power. The base station performs sliding matched filtering of the received signal to determine the presence of the preamble. The base station determines that the preamble is present and transmits an ACK to the mobile station if a maximum value of the sliding matched filtered sequence exceeds a certain threshold. In response to receiving an ACK of the preamble from the base station, the mobile station transmits the RACH message 180 to the base station.
FIG. 2 illustrates a RACH signal transmitter and receiver for an OFDMA system according to an example arrangement. Other arrangements may also be used. A transmitter may include a data coder 210 and an Inverse Fast Fourier Transform (IFFT) unit 220, whereas a receiver may include a Fast Fourier Transform (FFT) unit 240 and a data decoder 250. As shown in FIG. 2, in order to detect the RACH preamble, the FFT unit 240 converts a received time domain signal into a frequency domain sequence. Accordingly, a reference timing (with which the FFT is performed) may be needed. If the receiver performs a sliding FFT as a CDMA receiver (since the RACH signal is not synchronized with the receiver), then a calculation complexity may become overwhelming. To simplify the receiver configuration, the preamble of the RACH signal may include two symbols in the OFDMA system as shown in FIG. 3.
FIG. 3 illustrates a RACH signal transmission for an OFDMA system according to an example arrangement. A section defined by a predetermined time duration and a predetermined frequency band is allocated to a RACH since channels are divided by time/frequency sections in the OFDMA system. As shown in FIG. 3, a RACH signal includes two symbols. Thus, at least one section includes one full symbol, which includes the parts of two symbols. Accordingly, the receiver can perform FFT of the received signal for a section so as to detect the RACH signal without performing exhausted sliding FFT.
A SC-FDMA system may have a structure similar to the OFDMA system. The SC-FDMA system may be different in performing frequency domain conversion before an IFFT 320 for transmitting a signal substantially in a single carrier. The uplink channels may need to be synchronized within a limit in order to prevent interference noises between signals of adjacent frequency bands in both the OFDMA system and the SC-FDMA system. However, one symbol duration in the SC-FDMA system may be relatively short. Thus, interference problems for RACH signal detection may become more serious.